The invention relates to a method for igniting a fuel/air mixture in a combustion chamber of an engine.
In Diesel engines glow plugs are used to facilitate ignition, especially when the engine is cold. Glow plugs are usually heated to operating temperatures of 1,000° C. or more.
The combustion of fuel creates ions. This causes the conductivity of the gases in the combustion chamber to change significantly. Hence, information about the combustion process can by gained by measuring the electrical conductivity of the content of a combustion chamber. Such measurements are called ion current measurements. Special glow plugs can be used for ion current measurements, e.g., glow plugs disclosed in DE 100 15 277 B4 or U.S. Pat. No. 6,555,788B1.
Glow plugs for ion current measurements have a first terminal for applying a supply voltage for heating, which is provided by pulse-width modulation of an on-board voltage of the vehicle, and a second terminal for applying a measurement voltage of typically 40 V between pulses of the supply voltage. When the measurement voltage is applied to the glow plug, it is disconnected from ground by opening a switch. The measurement voltage then causes an ion current to flow from the glow plug through the gases in the combustion chamber to ground. The strength of the ion current is determined by the ion concentration caused by the combustion process.